In recent years, the tendency of processing and storing a large volume of data at a high speed has been increasing together with the development of digital technology. Therefore, a ferroelectric memory (FRAM (registered trademark)) using a ferroelectric film with a spontaneous polarization property as the capacitor insulation film has been developed in order to implement a non-volatile RAM where the write-in operation and the read-out operation are possible at a lower voltage and at a faster speed.
As for the FRAM, it is known to be effective to adopt, in place of a conventional planar structure, a stacked structure where a capacitor is formed directly above the contact plug formed on the drain of a transistor that forms a memory cell in order to reduce the cell area.
In an FRAM with a conventional stacked structure, the capacitor is formed directly above a tungsten (W) plug in such a manner that a barrier metal, a lower electrode, a ferroelectric film, and an upper electrode are layered on top of each other in this order, where the role of the barrier metal is to prevent the W plug from being oxidized.
Meanwhile, when the ferroelectric film receives physical damage during film formation for the upper electrode of the capacitor or damage from plasma or an etchant during processing, the crystal structure of the ferroelectric film is partially broken, and thus, the properties of the capacitor elements deteriorate. In order to restore the properties of the capacitor from the thus-deteriorated state, high temperature heat treatment is carried out in an oxygen atmosphere so that the oxygen restores the crystallinity of the ferroelectric film.
Furthermore, an aluminum oxide film that covers a capacitor is formed as a hydrogen permeation preventing film in order to protect the ferroelectric film from being deteriorated by hydrogen after the formation of the capacitor. Subsequently, heat treatment is carried out again in an oxygen atmosphere in order to restore the crystallinity of the ferroelectric film that has been damaged during the patterning of the ferroelectric film and during the formation of the hydrogen permeation preventing film before an interlayer insulation film is formed.
In order to improve the properties of the ferroelectric capacitor, it has been proposed to cover the ferroelectric capacitor with an interlayer insulation film, including a boron nitride (BN) film, of which the film formation is possible in accordance with a plasma CVD method that requires no application of a bias voltage (see Patent Document 1). In this case, no bias voltage is applied, which prevents hydrogen in the film forming atmosphere from entering into the device, and thus, the ferroelectric film can be prevented from deteriorating.
In order to deal with such a problem where an element that forms the ferroelectric film evaporates during a heat treatment process, it has been proposed to cover the ferroelectric capacitor with a double-structure film of reductive element permeation preventing film and an evaporating element compensation film (see Patent Document 2). In this case, the reductive element permeation preventing film prevents hydrogen from entering, and the evaporating element compensation film compensates the element that has evaporated from the ferroelectric film, and thus, the ferroelectric film can be prevented from deteriorating.